(bī'ə-lŏj'ĭ-kəl) An internal system that controls an organism's circadian rhythms, the cycles of behavior that occur regularly in a day. In mammals, the biological clock is located near the point in the brain where the two optic nerves cross. In many birds, the biological clock is located in the pineal gland. In protists and fungi, the individual cells themselves regulate circadian rhythms.

biological clock definition

The innate rhythm of behavior and body activity in living things. A twenty-four-hour cycle of body activity, which operates in some organisms, is called the circadian rhythm.

Note: Although the term biological clock refers to all innate timing mechanisms, it is often used when describing certain body functions that are subject to this rhythm, such as the loss of fertility with age.